1. Field of the Invention
The present invention relates generally to cement grouts and methods for use of the same. More particularly, the present invention relates to a pumpable cement grout for grouting of annular cavities.
2. Background Art
Much of the infrastructure of modern society is in need of repair or upgrading, or, at the least is in need of means for reducing the frequency of such repairs. For example, it is well known that a great many sewer systems throughout the United States and elsewhere are greatly in need of frequent repair.
One relatively new and effective technique that has been used for upgrading sewer systems involves relining existing concrete sewer lines with plastic liner pipe, such as high density polyethylene (HDPE) pipe. The plastic liner is inserted into the sewer line, and is pulled or jacked longitudinally through the sewer line into place. In order to fit within the concrete sewer line, the plastic liner pipe must have an outside diameter which is smaller than the inside diameter of the concrete sewer pipe, so as to provide clearance between the two. An annular cavity is thus formed between the pipe and liner. However, it is undesirable for the liner pipe to be very much smaller in diameter than the sewer pipe, lest the capacity of the upgraded sewer system be adversely affected. Accordingly, plastic liner pipes are typically installed so that there is adequate, but minimal clearance between the outside of the liner pipe and the interior surface of the sewer pipe; in a typical installation, the radial clearance (i.e. the radial extent of the annular gap) may range from about 3 inches to less than 1 inch.
Once the plastic liner pipe has been pulled or jacked into place, it is usually desirable to fill the annular space between the pipe and liner with grout to protect the liner from future damage. The potential for such damage may exist, for example, when the old concrete pipe is badly deteriorated and there is a danger that portions of the wall of the pipe may fail. A material which has been found suitable for such protective grouting is concrete grout which, in conjunction with the inexpensive plastic liner pipe, provides the potential for old sewers to be rehabilitated with minimal excavation and cost. However, the inside diameters of concrete sewer pipes typically range in size from about 60 inches down to 15 inches or less, and so direct access to the interior of the sewer line by personnel and construction equipment is usually difficult or impossible; this precludes the possibility of grouting of the pipe liner by means of the conventional cement grouting techniques which are used for backfilling large tunnels and similar structures, which typically involve boring holes through the wall of the tunnel at a multiplicity of points along the length of the tunnel, and then injecting cement grout through the holes and into each local area of the cavity about the tunnel.
Because of this inability to apply proven, conventional techniques, grouting of plastic sewer liners has been attempted by injecting a cement grout into the annular cavity at a first point along the pipe so that the grout flows longitudinally through the cavity towards a second point along the pipe. However, a very serious problem has been encountered when attempting to use this technique because the plastic liner pipe itself is typically unable to resist the significant external pressures which are exerted by the injected concrete grout. As noted above, the space between the plastic liner pipe and the existing sewer pipe is typically small; hence, it is typically very difficult to maintain a low grout pressure when pumping the grout longitudinally through the annular cavity. Unfortunately, the plastic liner pipe can easily collapse under even relatively low pressures, some pipes being unable to withstand external pressures as low as 3 pounds per square inch (psi). For example, a plastic liner pipe which is commonly used for lining sewer pipe is high density polyethylene (HDPE) pipe having a wall thickness of SDR 32.5 (where SDR is the ratio of outside diameter-to-wall thickness). HDPE pipe having a wall thickness of SDR 32.5, while able to contain significant internal pressures, has a tendency to collapse within one day if subjected to an external pressure in excess of 4 psi. Another, less commonly used size of HDPE pipe has a wall thickness of SDR 26, and tends to collapse when subjected to an external pressure in excess of 8 psi. Such external pressure maximums can easily be exceeded when attempting the longitudinal injection techniques described above using cement grouts generally exhibit fairly high viscosities, and relatively high injection pressures are thus required to force the grouts along the annular cavity. Furthermore, as the grout moves through the annular cavity, the grout tends to hydrate (i.e. bind or set up), particularly if the concrete sewer pipe and/or plastic liner are dry, thus compounding the difficulty of maintaining a low grout injection pressure; also, in many cases the plastic liner may be ribbed or corrugated, and the increased external surface area of this pipe tends to compound the hydration factor, as well as add increased friction.
These problems become critical when long distances are encountered between sequential injection points, in other words, when the individual runs to be grouted between access points are fairly long. This is often the case in conventional city sewers, where the distance between access points provided by manholes is often on the order of 300-500 feet or more. In some such cases, the contractors performing such grout work (using conventional cement grouts), have resorted to the expedient of drilling access holes vertically through the pavement and soil overlying the sewer pipe so as to provide additional grout injection points, which is obviously a costly and time-consuming makeshift approach. But, if the plastic liner collapses during the grout injection, the consequences may be catastrophic for the job. Not only is the flow of sewer water through the sewer line blocked by such a collapse, which may cause an overflow, but it may also be necessary to excavate and replace the entire section of sewer line in which the liner is collapsed, at great expense.
Accordingly, there exists a need for a grout material for grouting plastic sewer liners and the like, and a method for the use thereof, which is both effective and inexpensive, yet which reduces the possibility of collapse of the plastic liner pipe during grouting of the liner in the sewer line, particularly when using a ribbed or corrugated liner.